aconfig: add exported mode in c/c++ codegen

This commit adds exported mode to c/c++ codegen.

When the codegen mode is exported
1. only flags with exported: true will be generated
2. the generated getter should be in a read_write format and with
   default value as false, regardless the original permission and state
   of the flag

This change moves process parsed_flags funciton into codegen crate. The
process function is used to process parsed_flags based on the codegen
mode.

The template has also modified to keep readability.

Bug: 316357680
Test: atest aconfig.test aconfig.test.cpp aconfig.test.cpp.test_mode

Change-Id: I511e05ab93b07a04236055d956d1926f4ed89f36